As a method for measuring a resistivity of a silicon epitaxial layer, a direct-current four-probe method and a C-V (capacitance-voltage) method have been conventionally known.
According to the direct-current four-probe method, a tungsten carbide which is a cemented carbide is used for probes, four probes are vertically pressed against and brought into contact with a sample surface, a current I (A) is flowed through the outer probes, and a potential difference V (V) between the two inner probes is measured. A resistivity ρ of a wafer is calculated by using Expression (1).ρ(Ω·cm)=2πSV/I·Fw·Fr  (1,where S is a probe interval (e.g., 1 mm), Fw is a correction term based on a thickness of a wafer, and Fr is a correction term based on a diameter and a measurement position of the wafer.
A resistivity measuring instrument used in the direct-current four-probe method can perform calibration with the use of a standard wafer having traceability to resistivity standard reference materials (SRM) on seven levels, i.e., SRM 2541 (0.01 Ω·cm) to SRM 2547 (200 Ω·cm) provided by NIST (National Institute of Standards and Technology).
Each of SRM 2541 to SRM 2543 is obtained by lapping a boron-doped p-type CZ crystal into a wafer shape, and each of SRM 2544 to SRM 2547 is obtained by lapping a neutron-irradiated n-type FZ crystal into a wafer shape. The above-described standard wafer is also a bulk wafer obtained by lapping the CZ crystal or the FZ crystal into a wafer shape. The SRM selected to interpose a resistivity of the standard wafer is measured by a measuring instrument adopting the direct-current four-probe method to calibrate the measuring device, and the standard wafer is measured by the calibrated measuring instrument, thereby giving a value to the resistivity as a secondary standard value.
On the other hand, according to the C-V method, a Schottky junction is formed on a surface of a silicon single crystal wafer by using, e.g., a mercury electrode, and a depletion layer is spread in the silicon single crystal wafer by applying a reverse bias voltage to the electrode while continuously changing it, whereby a capacity is changed. Further, dopant concentration at a desired depth is calculated from a relationship between this reverse bias voltage and the capacity, and the dopant concentration is converted into a resistivity by using a conversion expression such as ASTM STANDARDS F723 or the like.
At present, there is no international standard or national standard (national measurement standard) rules or the like concerning resistivities obtained by the C-V method, and resistivity correlations are individually confirmed among trading companies as required. However, it has been already known that resistivities obtained by the C-V method and the direct-current four-probe method substantially have a correlation and, for example, Patent Literature 1 has a description that a resistivity provided by the C-V method and a resistivity provided by the four-probe method after an HF treatment substantially coincide with each other. Furthermore, Patent Literature 1 also has a description that a resistivity of an epitaxial layer in an epitaxial wafer can be calculated from a constant, sheet resistance of the epitaxial layer, and a film thickness of the epitaxial layer.